The present invention relates generally to the field of electronic device manufacture. More particularly, the present invention relates to the manufacture of integrated circuits using air gaps to reduce capacitive coupling between conductors in such devices.
Advances in integrated circuit technology have reduced the spacing between the metal lines on any given plane of an integrated circuit. Such reduced spacing between metal lines results in an increase in capacitive coupling between nearby conductive traces causing problems such as greater cross-talk and higher capacitive losses.
Low dielectric constant (“low-k”) materials (typically having dielectric constants of approximately 1.8 to 2.5) are being developed as a replacement for conventional dielectric materials used between conductors on a given layer and between layers. These low-k materials reduce capacitive coupling between the conductors as compared to conventional dielectric materials. However, low-k materials have not been used in all applications as they can possess severe processing, cost and materials problems.
The lowest possible, or ideal, dielectric constant is 1.0, which is the dielectric constant of a vacuum. Air is almost as good with a dielectric constant of 1.001. Attempts have been made to fabricate semiconductor devices with air gaps between metal lines to reduce the capacitive coupling between the electrically conducting members. The air gap forming techniques that have been developed have varying degrees of complexity but typically employ a material disposed between metal lines that is subsequently removed to provide an air gap. However, these techniques are not without problems.
U.S. Pat. No. 6,693,355 (Grove) discloses the use of a photosensitive material to form an air gap. According to this patent, a layer of photosensitive material is disposed on a substrate. A portion of the photosensitive material is then exposed to light to change the cross-link characteristics of the material. For example, when the photosensitive material is a positive tone material, exposure to light breaks the cross-links present in the material. A permeable second layer is then applied on the layer of photosensitive material. The exposed photosensitive material is then removed through the permeable second layer providing air gaps where the exposed photosensitive material was present. Unexposed photosensitive material (in the case of a positive tone photosensitive material) remains on the substrate under the permeable layer.
This approach presents problems when used with spin-on techniques. For example, the breaking of cross-links in the photosensitive material generally increases the solubility of the photosensitive material in most solvents used in spin-coating applications. Accordingly, if a permeable second material layer is applied by spin-coating, the solvent used for the second material layer will remove all or a part of the exposed photosensitive material. Alternatively, intermixing may occur between the uncross-linked photosensitive material and the second material layer resulting in a blending of the material. Yet another problem arises if the photosensitive material remaining on the substrate has a high dielectric constant as the effective dielectric constant of the total dielectric stack will be increased.
There is a continuing need for air gap forming materials and methods that can be easily applied to a substrate, that can be removed leaving little to no residue, and that can be used with spin-coating techniques.